For a long time it was assumed that it was some kind of fruit, or at least a nonsense word being used to represent a fruit, because that was the context is was used in in the book Charlie and the Chocolate Factory ("The snozzberries taste like snozzberries!"). Then, in 1979, people found out what it really meant when the author of Charlie and the Chocolate Factory wrote an adult novel that used the word in its proper context. Turns out, it's a penis.

"How did you manage to roll the old rubbery thing on him?"
"There's only one way when they get violent," Yasmin said. "I grabbed hold of his snozzberry and hung onto it like grim death and gave it a twist or two to make him hold still."
(from My Uncle Oswald)